The popularity of Apple Computer's iPod digital music players is helping it sell more Macs, but so far it hasn't been enough to spark a rise in the company's share of the personal computer market. According to research firm Gartner, worldwide PC shipments totaled 57 million units in the first quarter of 2006, representing a 13.1 percent increase over the same period last year. But in that time, Apple's share of the worldwide market slipped from 2.2 percent to a mere 2.0 percent.